How to Design Plastic Parts for Injection Molding?

Designing a plastic part for injection molding begins long before resin reaches the machine. The shape must perform reliably, fill evenly, cool predictably, and release without damage. Robert A. Malloy, author of Plastic Part Design for Injection Molding, offers a useful principle, paraphrased here: “Design the part for its function and for the process that will make it.” It sounds simple. It isn’t. A thick boss beside a thin wall, for example, can cool slowly and leave a visible sink mark. A sharp internal corner can concentrate stress. Small choices matter.

This outline introduces practical plastic component design decisions, from material selection and wall thickness to draft angles, ribs, and gate placement. It also explains how part geometry affects tooling, cycle time, appearance, and cost. Picture a thin-walled enclosure with a snap-fit lid: its tabs need enough flexibility, while nearby walls must remain stiff. That balance takes careful review. Design rules help, but they cannot replace testing. Resin grades behave differently, and mold conditions can expose weaknesses that drawings do not show. A prototype may still warp. A polished surface may reveal flow lines. These are useful warnings, not reasons to ignore sound design practice. The sections ahead provide a clear framework for reducing avoidable defects and making better-informed decisions with molders and engineers.

Table of Contents [Hide]

1 Select Resin and Set Tolerances Using Supplier Data and ISO 20457

2 Keep Walls Uniform—Often 1–4 mm, Depending on Resin and Part Size

3 Use 1–2° Draft and Rounded Corners to Support Ejection

4 Size Ribs at 40–60% of Nominal Wall Thickness to Reduce Sink Marks

5 Review Gates and Cooling; Typical Resin Shrinkage Ranges from 0.4–2.5%

Select Resin and Set Tolerances Using Supplier Data and ISO 20457

Resin choice sets the limits for a molded part. Compare supplier data for shrinkage, moisture absorption, flow, and recommended processing temperatures. A glass-filled resin may shrink less along the fiber direction, yet behave differently across it. That can shift a clip or mounting hole away from its intended position. Check data for the exact grade, not just the polymer family.

Use ISO 20457 as a framework for specifying tolerances on molded plastic parts. It helps connect tolerance requirements with material and manufacturing conditions, but it does not replace a conversation with the molder. Identify critical dimensions, such as a bearing seat or snap-fit gap, and mark less important surfaces with broader limits. Ask the supplier what variation is realistic for the selected resin, wall thickness, and tool design. Measure parts under agreed conditions; temperature and moisture can affect results.

Be practical. Tight tolerances can raise tooling and inspection demands without improving how an assembly works. A nominal hole size may look precise on a drawing, but resin shrinkage and part geometry matter too. Review sample parts, compare measurements with supplier capability data, and adjust the drawing before production. The first estimate may be wrong. That is useful feedback, not a failure.

How to Design Plastic Parts for Injection Molding? - Select Resin and Set Tolerances Using Supplier Data and ISO 20457

Resin family	Typical molding shrinkage*	Design considerations	Tolerance-setting approach	Supplier data to confirm
ABS, unfilled	Approximately 0.4–0.7%	Often selected for impact resistance and surface finish. Keep wall thickness reasonably uniform to reduce sink and warpage.	Use ISO 20457 to select the applicable tolerance group and assess each dimension by its size and function. Treat the drawing tolerance as a design requirement, not a guaranteed process capability.	Grade-specific shrinkage, flow direction effects, recommended processing window, and demonstrated capability for critical dimensions.
Polycarbonate (PC), unfilled	Approximately 0.5–0.7%	Useful where toughness and impact performance are important. Consider stress concentrations, chemical exposure, and the effect of molding stresses.	Apply ISO 20457 tolerance guidance to the part’s nominal dimensions and functional needs. Allow wider tolerances for non-critical features where assembly permits.	Grade-specific shrinkage, drying requirements, allowable molding conditions, and dimensional capability after conditioning.
Polypropylene (PP), unfilled	Approximately 1.0–2.5%	Shrinkage and warpage can vary with geometry and flow. Rib, boss, and wall design should limit local thick sections.	Do not derive tool dimensions from a single generic shrinkage value. Agree on the resin grade, tool-steel compensation, and tolerance plan with the molder.	Grade- and process-specific shrinkage, flow/transverse behavior, and warpage data for the intended part geometry.
Polyamide 6 (PA6), unfilled	Approximately 0.7–1.5%	Moisture absorption can change dimensions and mechanical properties. Define the condition in which dimensions are measured.	Set tolerances around the functional condition, including any specified moisture conditioning. Use ISO 20457 as the tolerance framework and document inspection conditions.	Conditioned and dry-state dimensional data, moisture-conditioning guidance, grade-specific shrinkage, and capability evidence.
Polyoxymethylene (POM), unfilled	Approximately 1.8–2.5%	Good dimensional behavior in many applications, but the relatively high molding shrinkage must be considered in tool design.	Identify critical mating and functional dimensions first. Agree on the ISO 20457 tolerance group and verify achievable tolerances through mold trials.	Grade-specific shrinkage, dimensional stability data, processing conditions, and measured capability for critical features.
Polybutylene terephthalate (PBT), glass-filled	Often approximately 0.3–0.8%; varies with glass content and flow direction	Fiber orientation can cause directional shrinkage and warpage. Gate location and flow path can strongly affect dimensions.	Review dimensions in both flow and transverse directions. Use ISO 20457 with the supplier’s directional data; confirm critical dimensions on molded samples.	Exact glass content, directional shrinkage, fiber-orientation effects, moisture and conditioning data, and process capability.

*Shrinkage figures are indicative ranges, not guaranteed material specifications. Actual values depend on the exact resin grade, part geometry, mold design, flow direction, and processing conditions. Obtain the current supplier data sheet and agree on measurement conditions before finalizing tool dimensions and drawing tolerances. ISO 20457 provides a framework for specifying tolerances for injection-molded plastic parts; the applicable tolerance group should be selected for the part and verified with the molding supplier.

Keep Walls Uniform—Often 1–4 mm, Depending on Resin and Part Size

Uniform walls help molten plastic cool at a similar rate. For many injection-molded parts, 1–4 mm is a useful starting range, depending on resin and part size. It is not a universal target. Thin walls may freeze before the cavity fills, while thick areas cool slowly and can develop sink marks or internal stress.

Check corners, bosses, and changes in section thickness. A thick boss joined directly to a thin wall can cool unevenly and pull the surface inward. Core out bulky features where possible, and use gradual transitions instead of abrupt steps. Ribs can add stiffness, but making them too thick may create visible marks on the opposite face. Small details matter.

A practical design review should compare the wall thickness across the whole part, not just inspect a single section. Mold-flow analysis or a test mold can reveal filling and cooling issues before production. Even a clean CAD model can hide a stubborn thick spot; it is easy to miss one. Adjust the target with the resin’s flow behavior and the part’s longest flow path in mind.

Use 1–2° Draft and Rounded Corners to Support Ejection

A molded part needs to release from the tool without scraping or sticking. Add 1–2° of draft to vertical walls, measured per side, so the part can move away from the cavity during ejection. A deep rib or textured surface may need more draft; the right amount depends on material, finish, and tool geometry. Even a small change can prevent drag marks along a tall wall.

Rounded corners help in two ways. They reduce sharp stress concentrations and let plastic flow more smoothly around a feature. As a practical starting point, use an inside radius near half the wall thickness, then keep the surrounding wall reasonably uniform. That matters. A thick corner can cool slowly and leave a visible sink mark, even when the draft looks correct. Check ribs, bosses, and pockets as well as the outer shape; they can create hidden undercuts or difficult release points. A clean CAD view is not proof that ejection will be easy. Review draft with a parting-line view, and confirm the details with the mold designer before tooling. These guidelines are useful, not universal. Test a prototype if the surface finish or fit is critical.

Size Ribs at 40–60% of Nominal Wall Thickness to Reduce Sink Marks

Ribs strengthen thin plastic walls, but a thick rib can create a visible sink mark on the opposite surface. Keep rib thickness near 40–60% of the nominal wall, a range commonly given in injection-molding design guidance, including Rosato et al.’s Injection Molding Handbook. For a 2 mm wall, that means a rib base around 0.8–1.2 mm. Small ribs matter. A 1.5 mm rib may look harmless in CAD, yet its thicker junction can cool more slowly and pull the outer face inward.

The handbook also notes that molding shrinkage varies by resin; common thermoplastics can shrink roughly 0.2–2.0%, depending on material and processing conditions. That variation makes the 40–60% rule a starting point, not a guarantee. Keep the rib junction smooth, add draft for ejection, and avoid stacking several ribs into one heavy mass. Check the part under real lighting, especially across broad, glossy panels. A subtle shadow can reveal a sink that was hard to spot on a screen. I would still test the chosen ratio with the actual resin and mold: geometry, gate location, and packing pressure can change the result.

Design note: A common starting guideline is to size ribs at 40–60% of the nominal wall thickness to help reduce sink marks. The values shown are calculated from that range; actual results depend on the material, part geometry, and molding conditions.

Review Gates and Cooling; Typical Resin Shrinkage Ranges from 0.4–2.5%

Injection-molded parts often shrink by 0.4–2.5%, depending on resin, geometry, and processing conditions. Treat this range as a starting point, not a fixed design allowance. A long, ribbed panel may warp differently from a compact block made from the same resin. Check the material supplier’s data, then confirm critical dimensions with molded samples.

Gate location and cooling layout both influence that result. Place gates where the melt can fill the cavity evenly, and avoid putting a gate on a visible surface when appearance matters. Uneven cooling can leave one side warmer, creating sink marks or bowing. Keep wall thickness as consistent as practical, and avoid thick junctions where ribs meet walls. Small details matter.

Tips: Use early flow and cooling analysis to compare gate positions. Keep cooling channels close to thick areas, but leave enough steel for strength. Measure parts after they have cooled and stabilized. Not always. A simulation may miss a real production variation, so review the first molded parts before locking dimensions. If a feature is unusually sensitive, allow room for adjustment rather than assuming the first tool setting will be perfect.